An arm rest device for seats of an automobile according to prior art employs a mechanism of a shaft lock system for adjusting an inclined/raised angle of an arm rest, as described for example in Japanese Utility Model Public Disclosure (KOKAI) No. 2-52643. Such a shaft lock system typically employs a coil spring, provided around a support shaft for permitting pivotal movement of the arm rest, as means for pivoting and positioning the arm rest, the coil spring having an inner diameter slightly smaller than an outer diameter of the support shaft. The support shaft with the coil spring fitted thereover is secured to the arm rest body. One end of the coil spring is secured to the arm rest body and the other end of the coil spring is rotated in a direction opposite to the direction in which the coils of the spring are wound, so as to increase the inner diameter of the coil spring to a diameter larger than the cuter diameter of the support shaft. By this, fastening force of the coil spring is reduced, so that relative movement between the support shaft and the coil spring is permitted, whereby the arm rest body may be pivoted. On the contrary, and when the other end of the coil spring is rotated in a direction in which the coils of the spring are wound, the inner diameter of the coil spring is reduced so as to increase a fastening force acting on the support shaft. By this, the support shaft and the coil spring are returned to a condition in which relative movement therebetween is not permitted. Thus, the arm rest body may be fixed at a predetermined stop position.